Light emitting diodes (LEDs) often are used for backlighting sources in liquid crystal displays (LCDs) and other displays. In backlighting implementations, the LEDs are arranged in parallel “strings” driven by a shared power source, each LED string having a plurality of LEDs connected in series. To provide consistent intensity and color emanating from the LED strings, each LED string typically is driven at a regulated current that is substantially equal among all of the activated LED strings.
Although driven by regulated currents of equal magnitude, there often is considerable variation in the bias voltages needed to drive each LED string due to variations in the static forward-voltage drops of individual LEDs of the LED strings resulting from process variations in the fabrication and manufacturing of the LEDs. Dynamic variations due to changes in temperature when the LEDs are enabled and disabled also can contribute to the variation in bias voltages needed to drive the LED strings with a fixed current. The lowest cathode voltage, or tail voltage, of all the activated LED strings typically must be sufficiently positive in order to properly regulate the currents through the activated LED strings. The variation in the voltage drops across the LED strings gives rise to the potential for the tail voltage of one or more LED strings to fall below the minimum voltage necessary for proper current regulation. Further, the output voltage provided by a power source driving the LED strings typically exhibits transient voltage droop when subjected to the pulsed current load that typically occurs in pulse width modulation (PWM)-based LED backlighting systems.
To account for both the variation in forward voltages between LED strings and the transient voltage droop in the output voltage, conventional LED drivers typically provide a fixed voltage that is sufficiently higher than an expected worst-case bias drop and transient voltage droop so as to ensure proper operation of each LED string. However, as the power consumed by the LED driver and the LED strings is a product of the output voltage of the LED driver and the sum of the currents of the individual activated LED strings, the use of an excessively high output voltage by the LED driver unnecessarily increases power consumption by the LED driver. Accordingly, an improved technique for driving LED strings would be advantageous.